Multi-frequency antenna



1955 G. B. HOADLEY ET AL MULTI-FREQUENCY ANTENNA Filed April 17, 1955 IN V EN TORS HOADLE'Y A 'IRNEY transformer section.

United States Patent 2,714,655 MULTI-FREQUEN CY ANTENNA Application April 17, 1953, Serial No. 349,414

Claims. (Cl. 250-) The invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment to us of any royalty thereon.

This invention relates to antennas and particularly to dual frequency antennas for use at very high radio frequencies.

In certain applications it is desirable to have an an tenna which is responsive to two or more different frequencies. In the prior art may be found numerous types of two-frequency antenna systems which may be used for transmission and reception. However, the prior art does not provide a multi-frequency antenna of the nonreradiating type for high radio frequency systems application. Accordingly, it is a principal object of this invention to provide an antenna which may be used for a plurality of frequencies, and which when used for receiving is of the non-reradiating type.

It is another object of this invention to provide a dualfrequency antenna system characterized by being of compact and simple construction.

For a better understanding of the invention, together with other and further objects and advantages thereof, reference is had to the following description, taken in connection with the accompanying drawings, in which:

Figure 1 is an isometric drawing illustrating the details of an antenna in accordance with the invention;

Figure 2 is a plan view of an antenna in accordance with the invention;

Figure 3 is an equivalent circuit diagram of the antenna shown in Figures 1 and 2.

In a study of problems involving the double-frequency method of detection it was determined that standing waves between a transmitter and a ground could cause considerable trouble by giving a decided height effect to the detector. In order to circumvent this difliculty it was necessary to provide a receiving antenna which would not reradiate. It was further determined that small antennas such as dipoles cannot be made in such a way as to eliminate reradiation. This led to the concept of an antenna with a large area which would match its impedance to that of air so that reradiation would not occur.

Referring now to Figure 1, parallel conducting plates 1 and 2 form the antenna proper and represent a variation of a parallel plate transmission line. Elements 3 and 4 are a pair of resonating sections connected in series at the end removed from the input. Element 5 is a dielectric member preferably of polystyrene dielectric. The combination 5, 3 and 4 provide the antenna with a 7 and 8 are conducting piston plates which are movable by means of knobs 9 and 10. A crystal detector 11 is connected in circuit with a utilization means for the received energy, across the plates 1 and 2 as shown.

The dielectric transformer 5 is of such thickness and so spaced from the crystal 11 that the impedances at both frequencies looking towards the termination shall be equal to the characteristic impedance of the line.

2,714,655 Patented Aug. 2, 1955 The transmission line is designed so that its characteristic impedance is so related to the impedance of the crystal 11, or such non-linear element as may be used, that the impedances at each frequency looking from the crystal toward the termination of the line shall be the conjugate complex which both resonant circuits are properly adjusted to their respective resonant frequencies.

The resonant circuits 2 and 3 are two short-circuited parallel plate transmission lines, each adjusted for resonance to its respective frequency.

It will be clear to those skilled in the art that resonant circuit 3 will present a maximum impedance to the frequency to which it is tuned; and that resonant circuit 4 will present a maximum impedance to the frequency to which it is tuned. At frequencies, other than those to which they are tuned, the circuits 3 and 4 will present a lower impedance, hence the maximum voltage across the crystal will occur at the resonant frequency of circuit 3 or, of circuit 4. The ment 5 providing an impedance match at both frequencies to prevent reradiation of energy.

Figure 2 shows somewhat more clearly the transmission line tuned circuits 3 and 4 and the location of the short-circuiting pistons.

Figure 3 is an equivalent circuit of Figures 1 and 2 and serves to illustrate the basic theoryof operation of the antenna in accordance with the invention. Since a short-cireuited line section such as 3 or 4 is the equivalent of a parallel circuit at its resonant frequency it may be replaced for purposes of clarity by a parallel resonant circuit such as 3 or 4'.

The operation of the antenna shown in Figure 1 will be made clear by reference to Figure 3. A quarterwave, short-circuited line behaves in the manner of a parallel resonant circuit offering a maximum impedance at its resonant frequency and relatively low impedance at other frequencies. Further, since a transmission line section such as 3 in Figure 1, has a very high Q, the frequency selectivity is quite good. In Figure 1, two quarter-wave line sections are utilized in a back-to-back arrangement, each being quarter-wave at a different frequency. Thus, if tuned-line section 3 is resonant at a frequency f1, and tuned-line section 4 is resonant at fre quency f2, they may be represented, by equivalent circuit elements 3' and 4 respectively of Figure 3. It will be apparent to those skilled in the art that these circuit elements for frequencies f1 and f that at. f1 circuit 3' offers a high impedance to the incoming signal, while circuit 4, not resonating at this frequency will offer little impedance to frequency f1; at a frequency f2, the reverse is true. Further, because of the dielectric transformer 5 of Figure 1, the line elements 1 and 2 are matched to circuits 3 and 4 at f1 and to assure optimum transfer of energy. Accordingly, detector means 11 will have impressed across it peak voltages at frequencies f1 and f2, with little or no voltage at other frequencies.

While we have illustrated a particular embodiment of the present invention, it should be clearly understood that operation on a pair of parallel conductmeans at one end of said positioned intermediate said third plate connected to said one of said pair of plates and said third plate, a second conducting piston means parallel conducting movably positioned between said other of said parallel plates and said third plate, wherein said third plate and said pair of plates form a pair of quarter-wave short circuited transmission line elements, means closely adjacent the open ends of said elements for detecting said incoming energy, said means connected across said parallel plates; and a dielectric impedance matching means positioned between said parallel plates at a point intermediate the open end of said pair of lines and said detecting means.

2. A two frequency antenna system for use at high frequencies comprising, a pair of spaced parallel conducting plate members of a predetermined length, a third plate member positioned intermediate said pair of plates in the same plane therewith and extend ing a portion of said length, conducting means for conmeeting together all ends of said plates, a first movable conducting piston member positioned between one of -;said pair of plates and said third plate, a second movable conducting piston member positioned between the other of said parallel plates and said third member; means connected across said pair of parallel plates closely adjacent said third plate for detecting received radio frequency energies, and a dielectric impedance matching means positioned between said pair of plates intermediate the open end of said parallel plates and said detecting means.

at one end and connected at the other end by a shortcircuiting member; a third conducting member extending from said short-circuiting member in the direction of said one end, said third member positioned intermediate said pair of parallel conducting members; a first conducting tuning member connected between one of said pair of members and said third member, a second tuning membe: connected between the other of said pair of members and said third member; output utilization means connected across said pair of parallel plates intermediate said open and short-circuiting means; and an impedance matching member located between said pair of plates at a point intermediate the output utilization device and said ,open end of said parallel conducting members.

4. A dual frequency antenna system for ultra high frequency radio waves, including an antenna portion comprising two parallel eonducting plate members open at one end, a signal responsive means positioned at the other end of said conducting plate members, said signal responsive means comprising two quarter wave shortcircuit line elements each tunable to a predetermined frequency and connected in series across said parallel plate members, an impedance matching means positioned across said parallel plate members at a point intermediate said signal responsive means and said antenna portion.

5. An antenna system according to claim 4 wherein said impedance matching means comprises a dielectric member,

No references 'cited. 

